1. Field of the Invention
The present invention relates to an ultraviolet-curing coating composition for cans, capable of forming a coating film which can be cured by irradiation with ultraviolet rays and which, when cured, is superior in coating film properties such as processability, adhesivity, hardness, hardness in hot water, etc., and superior in coating film appearance such as levelling property etc. and a process for producing a coated metal can by using the composition.
2. Description of the Related Art
Up to the present, as ultraviolet-curing coating composition, there are known cation-polymerizable coatings comprising a cation-polymerizable compound having an epoxy group(s) and a vinyl group(s) and a cation-polymerization initiator which generates a cation when irradiated with ultraviolet rays, radical-polymerizable coatings comprising a radical-polymerizable compound having a radical-polymerizable unsaturated groups and a radical-polymerization initiator which generates a radical when irradiated with ultraviolet rays, etc. However, the radical-polymerizable coatings, although having a feature of a relatively high curing rate, have problems in that they are insufficient in intimate adhesion to substrate and processability, are inferior in surface curing because the curing is inhibited by oxygen and particularly, when used for thin (2-8.mu.) film application, require an apparatus for nitrogen sealing or the like. Meanwhile, the cation-polymerizable coatings, although when compared with the radical-polymerizable coatings, having good adhesivity to substrate and good processability and requiring no apparatus for nitrogen sealing or the like, have problems in that they have a low curing rate and consequently are insufficient in film properties, particularly film appearance and retort resistance of film.
Moreover, any polymerization system has a problem in that the curability is insufficient when irradiated with ultraviolet rays at a low exposure.
Hence, the present inventors have made an intensive study with an aim of obtaining an ultraviolet-curing coating composition, capable of forming a coating film which can be cured by irradiation with ultraviolet rays at a low exposure, even in a thin film state, without requiring any facility for nitrogen sealing or the like and which, when cured, is superior in film properties necessary for a coating for cans such as processability, adhesivity, hardness etc., superior in film appearance such as levelling property etc., and also capable of forming a coating film which shows an excellent hardness in hot water, after heating after irradiation with ultraviolet rays. As a result, the present inventors have found out that the above-mentioned aim can be achieved by using a coating composition in which an alicyclic epoxy compound, a compound having an oxetane ring and a cation-polymerization initiator are compounded in specific amounts and completed the present invention.